1. Field of the Invention
The present invention relates generally to flow control valve apparatus and, more particularly, to a flow control valve apparatus provided with a pressure compensation function suitable for use in controlling the operation of a hydraulic actuator incorporated in a hydraulic machine such as a hydraulic shovel.
2. Description of the Related Art
A conventional type of flow control valve apparatus provided with a pressure compensation function, as disclosed in PCT publication No. WO83/01095, includes a housing having a main fluid passage provided with an inlet port and an outlet port, a main valve disposed between the inlet port and the outlet port for allowing or shutting off communication between the inlet port and the outlet port, a backpressure chamber formed between the housing and the valve member of the main valve for causing a fluid pressure to act upon the valve member of the main valve in the valve-closing direction, an auxiliary passage for providing communication between the backpressure chamber and the outlet port, and a pilot valve for selectively opening and closing the auxiliary passage to vary the fluid pressure in the backpressure chamber thereby operating the main valve. The main valve is formed as a seat valve, and the valve member of the main valve is provided with a plurality of slits which constitute an adjustable orifice by the cooperation with a cylindrical sleeve fixed to the housing, the backpressure chamber communicating with the outlet port through the slits. The auxiliary passage is formed in the housing, and a valve piston serving as a pressure compensation valve is disposed between the pilot valve in the auxiliary passage and the backpressure chamber. The pressure at the inlet side of the pilot valve or an equivalent pressure is conducted to one end of the valve piston through a first communication passage, while pressure at the outlet side of the pilot valve or an equivalent pressure is conducted to the other end of the valve piston through a second communication passage.
When an operating lever of the pilot valve is operated to open the pilot valve, the fluid in the inlet port flows through the adjustable-orifice type slits, the backpressure chamber and the auxiliary passage into the outlet port, to form a pilot flow. At this time, since the fluid flowing from the inlet port to the outlet port is restricted by the adjustable orifice, a pressure difference occurs between the inlet port and the backpressure chamber, and the fluid pressure in the backpressure chamber becomes lower than the fluid pressure in the inlet port. Thus, the valve member of the main valve is opened and the fluid in the inlet port flows into the outlet port through the main valve. The pilot flow rate at this time is determined by the set opening of the pilot valve, and the fluid pressure in the backpressure chamber is determined by the flow rate of fluid flowing through the slits, that is, the pilot flow rate. Therefore, the opening of the valve member of the main valve is finally determined by the set opening of the pilot valve, so that the flow rate proportional to the operation input of the pilot valve can be obtained in the main fluid passage.
In this state, when, for example, the fluid pressure in the inlet port rises and the differential pressure between the inlet port and the outlet port increases, the flow rate of fluid flowing through the main valve ends to increase and, at the same time, the pilot flow rate tends to increase. As a result, the differential pressure between the inlet pressure and the outlet pressure of the pilot valve increases. Since the differential pressure is conducted through the first and second communication passages to the opposite ends of the valve piston serving as a pressure compensation valve, the valve piston is displaced in correspondence with the increase in the differential pressure to restrict the pilot flow rate. Thus, the pilot flow rate decreases and the orifice effect of the slits is reduced. Therefore, the pressure in the backpressure chamber increases to cause the opening of the main valve to decrease. In consequence, the flow rate of fluid flowing in the main fluid passage is maintained at a predetermined relationship with the pressure increase in the inlet port, for example, kept constant to thereby provide pressure compensation.
However, such a conventional type of flow control valve apparatus encounters various problems such as complexity in structure, an increased number of steps of production and assembly, and an increase in production cost in that a high-precision pressure compensation valve including a valve piston is needed; in that a plurality of slits having high dimensional precision is needed to constitute an adjustable orifice in the valve member of the main valve; and in that it is necessary to provide a complicated fluid passage of passages including the first and second communication passages and the auxiliary passage in the housing. In addition, since pressure compensation is effected through the pressure compensation valve for controlling the pilot flow rate, it has been impossible to avoid the problem that satisfactory follow-up performance can be achieved with respect to variations in pressure.